I'm not sure this is going to be an issue, but I want to be prepared if it is.

I mailed out a book.  The day AFTER I mailed it I got a message from the recipient saying, "Oops, I my address forwarding expired.  If you haven't already mailed the book, could you mail it to my new address?  Otherwise, I'll try to get the post office to hold it." 

Well, I HAD already mailed the book, and let her know that.  But then I started thinking... I've already paid the postage on it and haven't received the credit for it, because it hasn't been delivered.  If I get it back (and even if the post office doesn't charge ME for it), even if I remail it to the new address, I'll be out the original credit, right? 

I checked the Help Center, but didn't see that issue addressed.  They say you can request an additional credit to re-mail it, but you haven't gotten the original one in the first place!  Am I misunderstanding something?

Hopefully, the receiver will be able to catch it at the post office and mark it received.  But if it's not, I want to make sure I understand this correctly. 

PBS is very firm that they will expect the other member to give you the credit. As they failed to update their address. Their fault.

From the Help Center (under change of address):

if books requested by you are  already "en route" to you when you move:

• You can make arrangements to collect them from the current inhabitants of the previous address if possible, OR
• You must contact your previous PO to arrange for special Media Mail forwarding. A regular "forwarding" or "change of address" order that works for letters does not typically work for Media Mail packages.
  • This may mean that the package will arrive at your new location postage-due, since Media Mail does not routinely include forwarding service - you will need to pay the postage due and claim the book.
• If you have requested books that have not been mailed yet, you can try to "catch" the sender using the PM button on the transaction, to provide your new address. 

It is the requestor's responsibility to provide a working address to senders.  If you do not keep your address current and correct on your account, you will need to give credits for (=mark received) any books that are sent to the wrong address, whether or not you are able to collect them, and whether or not they are returned to the sender.   Providing a dependable, correct address to senders is part of the honor system that helps keep this club working so well, and it is essential to maintaining membership.

Thanks Emily.  I read and read through the "My book was returned part" but didn't think to look through the club rules. 

Whew.... I feel better.  Thanks again!

The full text of the help doc cited by Emily is here
     What if I move or change my email address?
and additional related information can be found here
      How to change the mailing address on a request you have already submitted

Among other things, the latter document says

It is the requestor's responsibility to provide a correct, dependable address to senders.

• If you make an error and provide the wrong address, you should correct this as soon as possible, using one of the methods above.
• If the book you requested is returned to the sender because of an incorrect address, the sender is not obligated to re-send the book to you.
• If a requestor uses a nonworking address to request too many books, or makes this mistake too many times, this can jeopardize PBS membership.